In Hull depth transducer...no longer a fan.

[MikeP]

Several times in past discussions I have stated that my in-hull (shoot-through) backup depth finder (Raymarine ST40/Airmar P79 transducer) worked extremely well. It has always tracked within 2/10ths of a foot compared to my main through-hull/Garmin depth on the chart plotter.

Other folks have stated that in-hulls pretty much suck. But I wanted to try it some years back, figurring at worst I'd be out the price of a P79. And I have been extremely pleased with it...until this past week.

I was adding a second depth unit and while doing that, found that the new unit (Raymarine i40) would not reliably display depth. It would display, then flash for a while, then display, then flash etc. The ST 40 seemed to be working fine. So I concluded that the i40 was defective; based on my description of what was going on, the Raymarine tech agreed.

However, today the ST 40 started doing the same thing - not getting reliable depth.

I pulled the ducer wire back through the chase (PITA!) then popped the ducer off the mount that seals it to the hull and hung it over the side. I attached both depth displays in turn and they worked perfectly. So what was the difference between June, when the in-hull ducer worked perfectly and now?

The only thing that changed was the bottom was painted. So I have come around to the view expressed by others here that in-hulls are problematic for these boats, because, in this case, mine went from working perfectly to not working from what appears to be due to a coat of paint. Nothing else has changed. The in hull mount is still full of antifreeze, as it has been for several years.

I'm assuming that the ability of the ducer to shoot through the 1" hull must have been JUST adequate before the last coat of bottom paint and this coat made it too thick. At least that's all I can come up with.

They say that a better (more efficient) mount for a shoot through is via epoxy directly to the hull rather than using a "water box" with antifreeze or mineral oil. But I don't want to epoxy the ducer to the hull just to find out if it will work or not. So for now, I'll do without a back-up depth finder

Maybe next year I'll install another through hull transducer...

But the reality is I probably won't; hauling the boat just to do that isn't worth it to me and I don't have the cajones to try it in the water though I've heard of people doing it.

So, I guess I'd have to say that in-hulls MAY work fine - mine did for years - but may NOT after the bottom is painted again.

Again, I can't come up with anything that changed between June and now except a layer of paint.

 

[rsmith]

Higher metal content in the paint? In aircraft if the radome has a metallic flake paint stripe it will feedback the signal.

 

[SeaEric]

Its hard to imagine that a coat of paint would make the difference. But it is a boat. Anything is possible.

 

[MikeP]

I agree with you re a coat of paint but I can't find or think of any other change of any kind that is related...

Transducer works perfectly in the water. The water box that worked perfectly for years is still sealed and full of liquid, etc. all cables/wires are OK. I have never heard that trandsucers can get "weak" - just that they either work or they don't. But I'd sure be happy to discover that's the case and just buy a new P79.

According to Airmar, some of their in-hulls have no practical limitation shooting through FG hulls of the thickness of any Hatteras but that's more related to their larger units, not the P79.

 

[MikeP]

FWIW, I just got a call back from Airmar - the P79 is "rated" for hull thickness of up to 5/8" so the fact that it worked at all on the 53 indicates it works better than Airmar claims... They said the M260 would have no difficulty at all. BUT it's a $1000 unit and I don't need back-up depth that badly!

They confirmed that if it was at it's limit of capability, an additional coat of bottom paint could make the difference. Unfortunately, I guess, it did.

 

[jim rosenthal]

I have half a cartridge left of Pettit Flexpoxy, which might work well for this, if you decide to try gluing the transducer in.

Then again, you just saved a lot of money not buying a new autopilot.....

 

[MikeP]

"Then again, you just saved a lot of money not buying a new autopilot"

True!

Actually, I got the backup depthfinder working again - I moved the transducer from it's former position, pretty close to the inboard stringer in the port eng room bilge, to a new location about 3 feet outboard of that spot. Knocking on the hull seemed to indicate the hull is thinner as it moves away from the keel - at least it sounds thinner. So I attached the water box in that location, poured some antifreeze in, attached the transducer and so far the in-hull is working fine. Hopefully the slightly thinner hull there should solve the problem... at least until the next bottom paint which is at least 3 years away!